Monday, April 22, 2013

Object-Oriented Inheritance with JavaScript

JavaScript does not support an explicit inheritance operator the way Java or C++ do.
However, there are two ways to implement inheritance in JavaScript:

1. Using functions . This is the classical way.
2. Using Prototype. This is recommended and more powerful way of doing.  

Inheritance through Functions


1. Define super class.
2. Define sub class.
3. Assign the superclass object to one of the member method of the sub class.
4. Call the superclass constructor function inside the subclass.
/* Step 1 : define super class
*/
function superClass() {
this.sayHello = function(){alert('Hello SuperClass');}
this.sayMessage = function(){alert('Say Message SuperClass');}
this.name = 'superClass';
}

/*
Step 2: define sub class
a) assign the superclass object to one of the function object
b) call the superclass constructor inside the subclass
*/
function subClass()
{
//you can use any propertyname instead of parentClass eg superclass
this.parentClass = superClass;
this.parentClass(); // assigns the super class methods to subclass
this.sayHello = function(){alert('Hello SubClass');}
}

/* test */
function testSub() {
var sc = new subClass();
sc.sayHello();
sc.sayMessage();
}

Explanation


The explanation is pretty simple if you know the concept of “this” keyword in JS.
Remember that the ‘this’ keyword refers to the containing object.
So, when you call the following line, you have copied the entire superClass object to the subClass property
parentClass.
this.parentClass = superClass;
Then when you call the following line the constructor of the ‘superClass’ is fired.
this.parentClass();
Now, inside the superClass function you have the following line:
this.sayHello = function(){alert('Hello SuperClass');}
What does ‘this’ refer to?
Since the enclosing object of ‘superClass’ is ‘subClass’, the ‘this’ keyword refers to it’s enclosing object i.e.
“subClass” and not the ‘window’ object. Therefore all the properties of ‘superClass’ get copied to the ‘subClass’.

Inheritance through Prototype


This is the most suitable method of implementing inheritance in JavaScript.
1. The main advantage of this method is that the inheritance chain is dynamic i.e you can assign or remove   new methods and properties to the super class that becomes available to the child class automatically.
2. Also, JS natively supports prototypal inheritance and not class based inheritance.
3. We can only do method overriding and not method overloading.

Steps:

1. Create a super class.
2. Attach all the callable methods of super class to it’s prototype. Important step!
3. Create a sub class.
4. Assign the subclass prototype object to the super class instance. This creates inheritance.
5. Reset the constructor property for the sub class usingChildClassName.prototype.constructor=ChildClassName. This ensures that the objects created are
of the sub class and not the super class.
6. Call the super class methods using ClassName.prototype.methodName.call(this,parameters…). This will
work only for methods added via the prototype.

/* create a super class */
function Person()
{
this.sayName = function(){alert("Person::sayName()");}
this.name = "Person";
}
//create the callable methods in super using it's prototype
Person.prototype.sayHello = function(name){alert("Person::sayHello()");}

/* create sub class */
function Student()
{
this.name = "Student";
this.sayName = function(){alert("Student::sayName()");}
}

/* derive the sub class from super class by using it's prototype */
Student.prototype = new Person;

/*Reset the constructor property for the subclass using it's prototype.constructor property*/
Student.prototype.constructor = Student;
//add new methods to the super class which become available to all the sub classes automatically
Person.prototype.sayNew = function(){alert("Person::sayNew()");}
//call super class method
Student.prototype.callParentMethod = function()
{
//call parent methods this will work because sayHello is part of the object prototype
hierarchy
Person.prototype.sayHello.call(this);
//this NOT will work because sayName is NOT declared using the superclass prototype
Person.prototype.sayName.call(this); //error
}
function testProtoInherit()
{
var sc = new Student;
sc.sayHello();
sc.sayName();
sc.sayName('amit');
alert(sc.name);
sc.sayNew();
sc.callParentMethod();
}
testProtoInherit();
With Regards,
Animesh Nanda
Sr.Software Engineer | Photon Infotech
Bengaluru | Karnataka | INDIA.

Sunday, April 21, 2013

Javascript Prototype Object



Prototype is a builtin js property that is part of JS objects. It was introduced in JS version 1.1.
Prototype object can be added only to Function,String, Image, Number, Date and Array.
We can use prototype on the base class not on the objects derived from it.
The prototype object help you quickly add a custom method or member to an object that is
reflected on ALL instances of it.
Prototype object can only be used to add PUBLIC member and methods. It cannot add private
members or methods.
If the prototype object is used to override the the previously set property it will not override the
properties of already created objects. It will only affect the objects that have been created after
the prototype change.

Using prototype on custom objects:


var func = function(){};
func.prototype.foo = "bar"; //adds the property foo
alert(func.foo); //alerts undefined
func2 = new func();
alert(func2.foo); //alerts bar
func2.prototype.foo2 = 'ddd'; //Error : func2.prototype is undefined
Using prototype on builtin JS objects:

String.prototype.foo = "bar";
Image.prototype.foo = "bar";
Number.prototype.foo = "bar";
Array.prototype.foo = "bar";
You cannot use prototype on derived objects:
var str = new String();
str.prototype.foo = 'bar'; //Error : str.prototype is undefined




With Regards,
Animesh Nanda,
Sr. Software Engineer | Photon Infotech
Bengaluru | Karnataka | INDIA.